60 GEOLOGY OF TUK ISLE OF WIGHT. 



CHAPTER VI. 

 THE GAULT AND UPPER GREENSAND. 



1. The Gault. 



Inteoduction. 



The Gault, which rests quite conformably on the Carstone, may- 

 be described generally as a blue or bluish grey clay, more or less 

 sandy, and with minute spangles of mica. It contains little or no 

 calcareous matter, such proportion of this material as may have 

 been originally present having been converted into sulphate of 

 lime, which in the form of small crystals of selenite sometimes 

 occurs in considerable quantity. The fossils are few, and dis- 

 tributed at rare intervals. 



In thickness the Gault varies from 120 feet at Culver to 146 feet 

 at Blackgang, and 139 feet in Compton Bay. At Punfield, 

 where, however, it is difficult of measurement, it is about 111 feet 

 thick. In its upper part it becomes sandy and lighter in colour 

 than in the lower beds, so as to pass almost insensibly into 

 the Upper Greensand. The proportion of sand increases west- 

 wards in these passage beds, so that at Punfield the name of Gault, 

 as indicating a clay, becomes quite inapplicable. In the extreme 

 west (Black Down) the whole formation seems to pass into a 

 sand. 



Landslips. 



The Gault has received the name of the "blue slipper "* in the 

 Isle of Wight, from its tendency to give rise to landslips, or of 

 " Platnore," a name which was in former days applied to any close 

 black earthy stone or clay. The beautiful and romantic scenery of 

 the Undercliff or " Back " of the Island has been mainly caused by 

 the sliding of the Chalk and Upper Greensand over the unctuous 

 surface of the Gault clay, the tendency to slide being principally 

 due to a rather pronounced seaward southerly dip, and to the 

 outburst of springs at the junction oF the porous Upper Greensand 

 and impervious Gault. 



'* The term " slipper " is applied in tlie Islaud to any bed which gives rise to 

 hiudslips. 



